Using fluorinated plastics is a well known solution to protect metallic surfaces from corrosion by chemical agents. Their outstanding properties, due to their chemical structure, make them suitable for a wide range of applications in chemical, food, semiconductor and pharmaceutical industries.
Metal equipments used in the Chemical Process Industry (CPI) such as for example tanks, piping, reactors, pumps and valves built for example in brass, aluminum, bronze, stainless steel and carbon steel, are generally protected from the corrosion by using protective coatings based on polymers.
Pumps, tanks, reactors, impellers, tubes and other metallic devices and apparatuses are often coated with a fluoropolymer layer; well-known coating technologies allow obtaining fluoropolymer coating of the desired thickness in a single or more passes.
In the so-called CPI applications, the anti-corrosion coating shall guarantee the thermal, chemical and permeation resistance; with this aim, it is mandatory that said coating possesses no defects (holes, cracks and the like) that can be due to contaminations or processing problems, which might lead to the exposure of the underlying metal surface to the aggressive environment.
To statistically reduce the presence of this kind of damages, it is common practice to prepare such coatings in more than one pass, that is to say coating the metal surface with more than one layer of fluoropolymer. By doing so, should defects be generated when first coating the metal surface, an additional layer is supposed to cover said defects such as to obtain a coating having improved protective properties and increased lifetime.
Fluorinated polymers, especially those based on ECTFE (ethylene/chlorotrifluoroethylene copolymer) having a high Melt Flow Index (>10 g/10 min) are commercially used for their good chemical resistance as coating materials. Nevertheless, said materials generally require the use of a primer composition to improve adhesion to the metal; moreover, cohesion between multiple layers obtained in subsequent passes is generally reduced, so that delamination, blistering and bucking phenomena affect the efficiency of these coatings as anti-corrosive protective layers.
Among the various coating processing methods, those employing polymer dispersions in organic solvents or their aqueous latices and the electrostatic powder coating (EPC) can be mentioned.
Among said methods the electrostatic powder coating (EPC), wherein the polymer particles are electrostatically charged and deposited on the heated and earthed metal surface, have acquired a remarkable importance. Generally, the metal surface is previously cleaned and roughed, for example, by etching and sandblasting, to obtain a higher adhesion of the coating to the substrate and then heated at a temperature higher than the melting temperature of the polymer coating.
Such technique raises nevertheless great health and safety issues especially when coating internal surfaces of tanks or reactors of large dimensions, the operator is obliged to remain close to said hot surfaces (generally kept at temperature of more than 250° C.) generally with at least a part of his body inside the hollow bodies to be internally coated.
From a coating processing point of view, it would be thus preferable to use the fluoropolymers as liquid dispersion.
U.S. Pat. No. 5,502,097 (MITSUI DU PONT FLUORCHEMICAL) 26 Mar. 1996 discloses a water-based dispersion comprising:                a heat flowable fluororesin;        a dispersion medium having surface tension of <45 dynes/cm at 20° C., said medium comprising organic liquids, preferably alcohols;        a high boiling organic liquid having boiling point of 150 to 340° C., preferably ethylene glycol, glycerine, polyethylene glycol.        
U.S. Pat. No. 5,879,746 (DAIKIN IND LTD) 9 Mar. 1999 discloses aqueous dispersion compositions comprising:                a fluorine-containing melt-processable resin powder;        a water-soluble solvent;        a surfactant; and        water.        
The water soluble solvents suitable for preparing the composition of U.S. Pat. No. 5,879,746 (DAIKIN IND LTD) 9 Mar. 1999 can be chosen among three classes:                low boiling point (bp) (bp up to 100° C.) organic solvents;        middle boiling point (bp from 100 to 150° C.) organic solvents;        high boiling point (bp >150° C.) organic solvents.        
Among high boiling point organic solvents, alcohol solvents are preferred.
U.S. Pat. No. 6,124,045 (DAIKIN IND LTD) 26 Sep. 2000 discloses a coating composition comprising:                a perfluoropolymer (that is to say a fully fluorinated fluoropolymer);        a polyether-type surfactant, i.e. a surfactant comprising recurring units of ethylene and/or propylene oxide [—CH2—CH2—O— and/or —CH(CH3)CH2—O— type];        a liquid carrier, which is preferably a mixture of water with an alcohol, such as, inter alia, methanol, ethanol, isopropyl alcohol, ethylene glycol, diethylene glycol, triethylene glycol or glycerin.        
US 2004192829 (DAIKIN IND LTD) 30 Sep. 2004 discloses an aqueous dispersion comprising:                resin particles, like notably fluororesin or non-fluororesin particles;        a non-ionic surfactant, like notably a polyoxyalkylene alkyl ether-type surfactant;        water; and        optionally, a liquid organic compound having a hydrophilic group, preferably a polyol or an aromatic hydrocarbon solvent.        
EP 1416024 A (DAIKIN IND LTD) 6 May 2004 discloses an aqueous dispersion of fluororesin for coating comprising:    (A) fluororesin particles;    (B) a nitrogen-free high boiling point polyol having a boiling point of at least 100° C. and comprising at least 2 hydroxyl groups;    (C) a depolymerizable acrylic resin; and    (D) a non-ionic surfactant, like notably polyoxyalkylene alkyl ether-type surfactant and polyoxyethylene alkylphenol-type surfactant.
EP 335361 A (DAIKIN IND LTD) 4 Oct. 1989 discloses an electrodeposition coating composition comprising a fluorine-containing copolymer, a curing agent and an aqueous medium; thus, example 1 discloses a varnish comprising dimethyl ethanol amine and butyl cellosolve.
U.S. Pat. No. 6,140,408 31 Oct. 2000 discloses room temperature coalescable aqueous fluoropolymer dispersions which can comprise coalescing agents like notably (di)ethylene glycols alkyl ethers, and surfactants, like notably alkyl/aryl carboxylates ou sulfonates, e.g. TRITON XL-80N.
Dispersions of fluoropolymers of the prior art, especially dispersions of ECTFE, having suitable film-forming and adhesion properties, require thus the use of appropriate dispersing media, generally containing non negligible amounts of organic solvents which can have harmful environmental impact. Thus, while avoiding health and safety issues, these solutions add environmental concerns related to the presence of said solvents.
Increased attention to the environmental hazard prevention has thus generated a strong need for moving towards composition having reduced environmental impact, that is to say comprising less and less solvent, while maintaining or even improving coating performances.
Another characteristic of anticorrosion layers is the low permeability against chemical substances. Particularly critical for fluoroplastics is the water vapor resistance. When the permeability to water vapor under severe operating conditions is too high, said vapor can penetrate the anti-corrosion coating and accumulate between the anti-corrosion layer and the metal surface. As a consequence, the protective coating is progressively delaminated from the surface to be protected, and buckling and blistering phenomena can considerably reduce its effectiveness and lifetime.
There is thus a need in the art for fluoropolymer compositions that provide both for good adhesion to metal surfaces and for good cohesion between successive layers, with no need of adhesive, tie-layers or primers, and that can be processed into coatings on metals with more health and safety-friendly techniques, which possess limited environmental impact and which exhibit improved mechanical properties and water vapor resistance, yielding improved protective performances and lifetimes.